Wednesday, March 16, 2011

cisco packet switching order

This Lab setup i was trying to understand the Routing TCP/IP vol 1 - cisco packet switching order.



In this setup if we enable the debug ip packet on the R1 and see whether packet transferred between Host 1 &  Host 2 ? lets ping from Host2 to Host1
mm execpt some broadcast packet nothing in the debugging output why ?

R1#debug ip packet
IP packet debugging is on
R1#
*Mar 1 00:15:00.823: IP: s=0.0.0.0 (FastEthernet0/0),
d=255.255.255.255, len 576, rcvd 2
R1#
*Mar 1 00:15:03.835: IP: s=0.0.0.0 (FastEthernet0/0),
d=255.255.255.255, len 576, rcvd 2
R1#
*Mar 1 00:15:06.839: IP: s=0.0.0.0 (FastEthernet0/0),
d=255.255.255.255, len 576, rcvd 2
R1#
*Mar 1 00:15:29.907: IP: s=0.0.0.0 (FastEthernet0/0),
d=255.255.255.255, len 576, rcvd 2
R1#
*Mar 1 00:15:32.911: IP: s=0.0.0.0 (FastEthernet0/0),
d=255.255.255.255, len 576, rcvd 2
R1#
*Mar 1 00:15:35.911: IP: s=0.0.0.0 (FastEthernet0/0),
d=255.255.255.255, len 576, rcvd 2


answer is
ip packet debugging only shows the packet transferred through packet switching.

Lets try further:
Lets change the processing to process switching on the R1 fa0/0 which is facing 192.168.1.2 Host. to disable cef we can use following command.


R1(config)#int fa0/0
R1(config-if)#no ip route-cache ?
cef Enable Cisco Express Forwarding
flow Enable Flow fast-switching cache
policy Enable fast-switching policy cache for outgoing

packets
same-interface Enable fast-switching on the same interface
R1(config-if)#no ip route-cache cef
R1(config-if)#no ip route-cache

Lets try to ping from 192.168.3.2 to 192.168.3.1


R1#
*Mar 1 00:36:30.131: IP: tableid=0, s=192.168.1.2 (FastEthernet0/0), d=192.168.3.2 (FastEthernet0/1), routed via FIB
*Mar 1 00:36:30.131: IP: s=192.168.1.2 (FastEthernet0/0), d=192.168.3.2 (FastEthernet0/1), g=192.168.2.2, len 84, forward
R1#
R1#
R1#
R1#


mmm .. only one packet shown here others ? above output shows two behaviors:

when the inbound is Process switching and outboud CEF / fast switching the resulting switching method will be Fastswitching that is the output shown here the first packet process switched as the expected behavior for fast switching

there is not packet shown as the src 192.168.3.2 because that stage the processing will happen via CEF( if inbound CEF & outbound process / Fast switching method CEF)

summary :

1) If the inbound is CEF switching method will be CEF
2) if the inbound is Process and the outbound is CEF / Fast resulting switching will be Fastswitching
3) if the inbound is Fast and outbound is CEF resulting will be Fast
4) if the inbound is Fast and outbound is Process resulting will be Process

There are exemption list we need to consider.

Reference : Table 3.1 Chapter 3 - routing tcp/ip vol 1 2nd edition.

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